Remove lazy join from ipallocationpools

The combiniation of slack addresses for ipv6 and poor handling
of ipv6 ipavailabilityranges in neutron makes many calls to
neutron's API(like net-list) extremely slow.
To fix this we need to remove the lazy join from the
IPAllocationPool model.

Change-Id: I124ff2c7becf92cfaecbd26d866d2bbdac465408
Signed-off-by: Rosario Di Somma <rosario.disomma@dreamhost.com>